Features

This device is designed to control brushed DC motors up to 40 V and 18 A
(720 Watts, 0.97 HP) continuous operation. It is ideal for adapting
CNC machines such as routers, mills and 3D printers to
Servo operation with low cost
DC motors. It is pin compatible with the
MassMind BOB PID Servo Control. It can
also be used as the power stage of a mobile robot., DC-AC inverters, and
for driving Peltier modules.

Open Loop Control: For basic open loop control, supply a PWM signal
with a duty cycle for the desired speed to the PWM (aka STEP) pin and set
the direction on DIR, while enabling the driver with a high value on ENB.

MOSFET driver

A DRV8701^ full
bridge mosfet driver provides the capabilities to drive the MOSFET gates
at an appropriate voltage with acceptable switching speed, while also protecting
them from being damaged from heating due to insufficient gate voltage,
shoot-through, etc.

It also controls an indicator LED for signaling the fault modes the device
is able to detect, such as motor overcurrent and driver undervoltage.

Finally, the driver also performs a current limiting function to avoid damaging
the transistors. The current level is set via a low pass filtered PWM signal
or by a onboard trimpot.

Please note: Catch diodes, FET gate resistor, FET gate zener clamp diodes
are NOT needed in this design. The internal diodes of the FETs are fast enough
to catch any spikes, switching currents are set by one resistor on the board
(not individual resistors for each FET), and the voltage clamps are provided
with the gate driver.

MOSFETs in H bridge configuration

The device utilizes four IRFB7546 MOSFETs. These transistors have a breakdown
voltage of 60 V and a RDSon of 6.0 mOhm typically (a worst case resistance
of 14.1 mOhm at 175 °C Tj) with a Vgs of 10 V.

Current Limiting

Current limiting is accomplished by the driver by turning off the appropriate
transistors by a voltage comparator sensing the voltage drop across RSENSE
and comparing it to the voltage on LIM pin.

For setting the current limit using the onboard potentiometer one needs to
measure the voltage between ground and the wiper pin. Then, one can calculate
the current with the following expression:

I = 10[ohm-1]
Vpin - 0.5[A]

or

Vpin = (I + 0.5) / 10

For example, to get the maximum continuous current of 18 A, The wiper pin
needs to measure 1.85 V. 18 + 0.5 = 18.5 then divide by 10 = 1.85

Overtemp Protection

The current sensing resistor on the PCB reduces the current applied when
the PCB temperature rises, preventing runaway and limiting max current when
approaching the top end of capacity. In extreme cases, when extreme currents
are applied, the resistor will act as a fuse, cracking and shutting down
the unit, protecting the drivers.

PMinMO Connector (Option)

Connector

Pin

Signal

Description

JP1 -Bottom
(all even pins
are ground)

1

ENB

Enable

3

DIR

Direction

5

PWM

PWM signal for motion

7

GND

Ground pin for the board.

9

5V

Positive supply input for the board. PWM signal for off-board current
limit

Steps

1. [ ] Install JP1 shrouded header with the notch towards the inside of the
board. (or install a 4 pin screw terminal in the same space)

2. [ ] Install MTR and PWR screw terminals.

3. [ ] Align the board with the heatsink and verify the position of mounting
holes and the placement of Q1-4 MOSFETs. Mark, punch, drill, and
tap holes for #4-40 bolts into the heatsink.

4. [ ] The power drivers will run across the back of the board, on 0.475"
centers, about 0.25" from the back and with the first one centered at 0.280"
from the left edge. The 4 power transistors Q1-Q4 MUST be insulated from
the heatsink. This means the transistor bodies and metal tabs are NOT
electrically connected to the heatsink.

Use the clear Mica insulating washers, these go between the transistor body
and the heatsink (see '1' in the diagram above). Use heatsink compound on
both sides of the Mica washer, this only requires a tiny smear in the middle
(which will squish flat when tightened down).

There is also a plastic insulator "shoulder washer" (see '2' in the diagram)
this stops the metal screw touching the metal tab of the transistor. No heatsink
compound is required on the screw.

After the transistors are mounted you should check them with a multi-meter
(ohm meter or continuity tester) to make sure the transistor tab is NOT CONNECTED
to the heatsink. Likewise the transistor tab should NOT be connected to the
metal screw.

Once the mechanical mounting is done, solder the MOSFETs to the PCB.

5. [ ] Install C7 long lead in the square hole, and the stripe side towards
the round hole. It can be installed all the way down on the PCB for physical
stability, or bent over toward the RSENSE resistor, and secured with a bit
of hot glue or double back foam.

Mechanically, the PCB is 1.97" wide by 1.57" deep, with mounting holes, as
measured from the bottom left corner, at:

0.11", 0.11"

1.85", 0.11"

0.11", 1.45"

1.85", 1.45"

Installation / Connection

Power: The device comes with two screw terminals. The one
marked with the MOTOR-A and MOTOR-B labels should be connected to the two
cables for powering your motor. The other terminal, marked V+ and V- should
be connected to a power supply of the correct voltage, ensuring the correct
polarity of the connection: Ground to V- and the positive supply to V+.

Terminal Block: An 0.2" spacing, 4 position terminal block
can be installed for the signal header as an option. In this case, the LIM
pin must be patched to ENB so the current limit is provided on board and
is adjusted via the trim pot or you can hort the SJ1 jumper (lower left corner
of the board, under the current adjust pot) to provide logic power to the
current limit (LIM) signal. The signals will be:

ENB (nearest power / motor connector) the enable signal for the driver,
active high. When the enable signal is present, the PWR LED will light (assuming
the motor power supply is already powered). If the enable signal is not present,
the PWR LED, and every MOSFET on the board will be turned off.

PWM signals the motor to turn or stop, active high. PWM on this signal
varies how hard the motor will try to turn. When in low state, the driver
brakes the motor through the low side MOSFETs.

1. ENB the enable signal for the driver, active high. When the enable
signal is present, the PWR LED will light (assuming the motor power supply
is already powered). If the enable signal is not present, the PWR LED, and
every MOSFET on the board will be turned off.

3. PWM signals the motor to turn or stop, active high. PWM on this
signal varies how hard the motor will try to turn. When in low state, the
driver brakes the motor through the low side MOSFETs. (on a standard PMinMO
connector, this would be DIR)

5. DIR signals the direction of turn, forward or reverse. (on a standard
PMinMO connector, this would be STEP)

7. Ground (all even numbered pins are also ground)

9. LIM a current limit signal. On the standard PMinMO, this is +5,
but the driver get's it's own logic power from motor power. The 5V applied
to this pin will instead be use with the onboard trimpot for setting
current limit. Or you can implement your own current control by providing
your own voltage to this pin. It is possible to use a PWM signal to adjust
the current limit, as a low pass filter is present on the board. You can
also short the pads of SJ1 on the PCB to provide LIM from the onboard logic
power; be aware this will back drive whatever you connect to pin 9 when the
driver is enabled.

Operation

Safety Checklist

Things can move unexpectedly. Keep all body parts out of any possible path
of movement.

40 volts can easily kill you given a good contact and the right path. Do
NOT touch live wires.

Drivers, Motors, Power supplies, and the wires between them can get very
hot. Take the time to check with the back of your hand, then tap, then touch.

First Movements

Note that the LED on the board will NOT light, and nothing will move,
even when motor power is applied, until the ENB (enable) signal is
high.

Ensure ENB is low before continuing then apply motor power. Check
that there are no arcs, pops, cracks, or fires. The LED should still be off.
If the LED comes on, check that ENB is low (0 volts).

Supply a direction signal to DIR and PWM at 0 percent duty cycle. i.e. 0
volts DC. Either supply +5 to the LIM pin 9 of the PMinMO connector or short
the SJ1 pads if you are using terminal blocks to provide a current limit
reference.

With PWM off set ENB high, and check that the motor does not move.
The LED should light on the board.

Apply a low duty cycle PWM signal and verify that the motor moves making
note of the direction.

Set PWM off, reverse direction (low to high or high to low) then apply a
low duty cycle PWM again and verify the motor moves in the opposite direction.

Adjust Current Limit

To avoid burning out a motor, adjust the onboard current limit system to
provide no more power than the motor can manage. The voltage at the current
limit test point near the should be set while the unit is running at full
speed. Start with the desired maximum wattage and divide by your motor supply
voltage to find maximum current. Then use that current to calculate the correct
setting using this formula:

Vpin = (I + 0.5) / 10

For example, a 350 watt motor using a 35 volt supply should be limited to
10 Amps maximum current. 10 + 0.5 is 10.5, divided by 10 is 1.05, so the
pot should be set to about 1 volt.

Hint: For anything over 10 amps, just set Vpin to the current divided
by ten it's close enough.

Procedure:

Start with the current pot all the way down... Counter Clockwise. Note:
on the v1.1 unit the trimpot does not have a stop, and will rotate continuously.
The flat spot indicates position, and is at minimum when towards the edge
of the board.

Apply 100% PWM... The motor should NOT turn at full speed. Connect the ground
lead of your multimeter to ground in the terminal block, and the positive
lead to the small round pad just above the pot, near "C6".

Slowly turn pot up (Clockwise) until voltage is the calculated Vpin. Note:
The LIM signal pin 9 on PMinMO connector must be at +5 volts or SJ1 must
be shorted to provide the max current limit signal.

Shut down PWM.

Heat

Be very careful to monitor the heat of the drivers for at least 20 minutes
during normal operation. It can take time for heat to build up, and excess
heat can fry the drivers. It's probably a good idea to monitor the motor
as well.

jmyerSPAM at spammyermountain.com
asks: " Is there a variant of the DRV8701s
H Bridge driver design that could be build to accommodate a 90V DC Motor?
Perhaps by swapping out the Mosfets?"
James
Newton of MassMind replies: That generally doesn't work well because
the electronics driving the power FETs is carefully tuned to the ones in
use.+

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